Introduce support for Traefik-native external middlewares referenced by resources.
- Database: add resource_external_middlewares table in migrations and ensure creation in post-migration updates.
- API: add handlers and routes to assign, remove and list external middlewares; include external_middlewares field in GetResources/GetResource responses (comma-separated name:priority:provider entries). Handlers validate resource status, use transactions, and log errors.
- Services: ConfigProxy now loads external middleware refs, merges them with internal middlewares sorted by priority when building resource config.
- UI: Resource detail component, API client, store and types updated to expose listing, assigning and removing external middlewares with UI controls and confirmation modal.
- Tests: add unit tests for assign/remove/list handlers and inclusion on GetResource.
This enables referencing middlewares defined outside MW-manager (e.g., Traefik dynamic config or plugins) and honors priority/provider metadata.
Update TestDataSourceConnection to support requests with an empty body by loading the existing data source config if available. Returns 404 if the data source is not found, otherwise proceeds as before.
